The present invention relates to a suspension control apparatus for a vehicle, such as an automobile or a rail carriage.
In terms of suspension control, an ideal standard is defined by the so-called skyhook theory, whereby a spring is provided between a sprung mass and an unsprung mass and one damper is provided between the sprung mass and a fixed point in air above the sprung mass. By this arrangement, a damping force equal to the product of a vertical velocity of the sprung mass and a damping coefficient of the damper is applied to the sprung mass.
However, in actual practice, it is not possible to fix a damper at a point in air above a sprung mass, and such a damper must be positioned between the sprung mass and the unsprung mass. In this arrangement, in order to obtain substantially the same suspension characteristics as those which would be provided by the ideal damper positioning defined in the skyhook theory, it is necessary for a fluid to be supplied to or discharged from a damper, to thereby enable frequent changes in the length of the damper. To effect such fluid control, a large capacity fluid supply system is required. Such a control provided with a large capacity supply system is referred to an active suspension control.
In Unexamined Japanese Patent Application Public Disclosure (Kokai) No. 5-330325 (corresponding to U.S. Pat. No. 5,533,597), the inventors have proposed a suspension control apparatus which provides suspension characteristics analogous to those which would be provided by a damper obtained by the skyhook theory, without the need for a large capacity fluid supply system. In this apparatus, vibration of a vehicle body is suppressed by adjusting a damping coefficient (hence a damping force) of a variable damping coefficient type shock absorber. This control is referred to as a semi-active suspension control.
In the suspension control apparatus of Kokai No. 5-330325, vertical acceleration of a vehicle body is detected and an absolute vertical velocity of the vehicle body is determined by obtaining an integrated value, based on a signal which corresponds to the detected acceleration. Then, a damping force based on the skyhook theory is calculated from the absolute vertical velocity, and a semi-active shock absorber (a variable damping coefficient type shock absorber) provided between the vehicle body and an axle is controlled, so as to generate a damping force equivalent to that of the skyhook damper. The method of this control will be explained later in detail.